sedimentary

Fossilized Coral (Petoskey Stone type)

Hexagonaria percarinata (Fossilized Rugose Coral)

Hardness: 3-4 (Mohs scale); Color: Gray to beige; Luster: Matte to waxy; Crystal structure: Calcite/Aragonite replacement; Features: Distinctive hexagonal 'star' or 'honeycomb' patterns on the surface when wet or polished.

Hardness: 3-4 (Mohs scale)
Color: Gray to beige
Luster: Matte to waxy

Identified April 4, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed during the Devonian Period (approximately 350 million years ago) from the skeletons of ancient coral colonies in warm, shallow seas. Over time, the coral was buried and the skeletal structure was replaced by minerals like calcite.

Uses & applications

Used primarily for jewelry (cabochons), tourist souvenirs, decorative lapidary work, and as a popular collector's item for amateur geologists.

Geological facts

This specific type of fossilized coral is the state rock of Michigan. The name 'Petoskey' comes from an Ottawa Chief, Pet-O-Sega, which means 'Rays of the Rising Sun,' referring to the sunburst pattern seen in the fossil.

Field identification & locations

Identify in the field by looking for gray stones on shorelines that reveal a 'honeycomb' pattern when dipped in water; dry specimens often look like ordinary gray pebbles. Most common in the Great Lakes region, especially northern Lake Michigan.

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